The present invention relates generally to noncontact gauging systems. More particularly, the invention relates to an apparatus system and method for calibrating noncontact gauging systems.
Demand for higher quality has pressed manufacturers of mass produced articles, such as automotive vehicles, to employ automated manufacturing techniques that were unheard of when assembly line manufacturing was first conceived. Today, robotic equipment is used to assemble, weld, finish, gauge and test manufactured articles with a much higher degree of quality and precision than has been heretofore possible. Computer-aided manufacturing techniques allow designers to graphically conceptualize and design a new product on a computer workstation and the automated manufacturing process ensures that the design is faithfully carried out precisely according to specification. Machine vision is a key part of today's manufacturing environment. Machine vision systems are used with robotics and computer-aided design systems to ensure high quality is achieved at the lowest practical cost.
Achieving high quality manufactured parts requires highly accurate, tightly calibrated machine vision sensors. Not only must a sensor have a suitable resolution to discern a manufactured feature of interest, the sensor must be accurately calibrated to a known frame of reference so that the feature of interest may be related to other features on the workpiece. Without accurate calibration, even the most sensitive, high resolution sensor will fail to produce high quality results.
In a typical manufacturing environment, there may be a plurality of different noncontact sensors, such as optical sensors, positioned at various predetermined locations within the workpiece manufacturing, gauging or testing station. The workpiece is placed at a predetermined, fixed location within the station, allowing various predetermined features of the workpiece to be examined by the sensors. Preferably, all of the sensors properly positioned and should be carefully calibrated with respect to some common fixed frame of reference, such as a common reference frame on the workpiece or at the workstation.
Keeping sensors properly positioned and calibrated is more easily said than done. In a typical manufacturing environment sensors and their associated mounting structures may get bumped or jarred, throwing the sensor out of alignment. Also, from time to time, a sensor needs to be replaced, almost certainly requiring reorienting and recalibrating. Quite simply, sensor positioning, alignment and calibration is a fact of life in the typical manufacturing plant.
The problem with sensor positioning, alignement and calibration is the time required. Invariably, the entire manufacturing assembly line for a given part must be shut down and the workstation cleared, so that the sensor may be positioned, aligned and recalibrated. In some instances this entails placing a highly accurate (and very expensive) full-scale model of the workpiece in the workstation. This highly accurate master part is sometimes called a "golden part" or "body in white." The master part is placed in careful registration with the external coordinate system of the workstation and then each sensor is trained on its assigned feature (such as a hole or edge). Once positioned, the sensors are locked into place and calibrated and the master part is removed. Only then can the assembly line be brought back online.
As an alternative to using a master part, it is possible to calibrate the gauging sensor by attaching a target to the sensor and illuminating the target using a plane of structured light produced by the sensor. A pair of optical sighting devices, theodolites, are placed at different vantage points within the workspace. The theodolites triangulate on the illuminated target to provide an independent reading of the position of the target. The theodolites are placed at carefully prescribed locations relative to the external reference frame. With the gauging sensor projecting structured light onto the target, the theodolites are manually aimed at the lighted targets and readings are taken. The respective readings of the theodolites and the gauging sensor are coordinated and translated to calibrate the sensor relative to the external reference frame. It is a trial and error process. If the sensor needs to be reoriented (as is often the case), the theodolites must be manually retrained on the target after each sensor position adjustment. For more information on this calibration technique, see U.S. Pat. No. 4,841,460 to Dewar et al.
Whereas both of the aforementioned calibration techniques do work, there is considerable interest in a calibration technique that is quicker and easier to accomplish and that eliminates the need to rely on expensive master parts or expensive theodolite equipment. To this end, the present invention provides a calibration system that can be used in a matter of minutes, instead of hours, and without the need for precisely manufactured master parts or theodolite equipment. One of the major advantages of the invention is that it allows the calibration of a sensors to be checked or realigned between line shifts, without requiring the line to be shut down for an extended period.
The calibration system employs reference indicia that are disposed in fixed relation to the external reference frame of the manufacturing or assembly zone or gauging station. These reference indicia may be simple light-emitting diodes that are disposed at predetermined fixed locations. A calibration sensor array is positioned at a vantage point, typically above the gauging station, so that the reference indicia are within the sensor array's field of view. If desired, the calibration sensor array can be permanently or semipermanently mounted at a suitable vantage point within the workpiece manufacturing or assembly station. This is not a requirement, however, as the sensor array is comparatively lightweight and can be quickly placed in position when it is time to calibrate one or more of the feature sensors.
The calibration system further employs a portable reference target that is placed within the observation field of the sensor array and also within the sensing zone of the feature sensor to be calibrated.
The presently preferred portable reference target is a tetrahedron framework having light-emitting diodes at the vertices. The tetrahedron framework provides at least three noncolinear and noncoplanar geometric structures (e.g., straight edges) that are illuminated by structured light emanating from the feature sensor. These noncolinear geometric features provide the feature sensor with unambiguous spatial data for measuring the spatial position and attitude of the target. The light-emitting diodes at the vertices of the target comprise at least three point sources of light that may be switched on and off, or otherwise controlled, to provide unambiguous spatial positioning data to the sensor array. The light-emitting diodes comprising the presently preferred reference indicia within the fixed reference frame may be switched on and off or similarly controlled.
The system further includes a coordinate translation system for coordinating the readings from the sensor array and from the feature sensor. More specifically, the translation system is adapted for coupling to the sensor array to the feature sensor to collect data from various light-emitting and light-reflecting structures read by these sensors. The translation system establishes a first relationship between the reference frame of the sensor array and the external reference frame with which the reference indicia are associated. The translation system also collects data from the portable reference target as viewed by both sensor array and feature sensor and establishes a second relationship between the array reference frame and the feature sensor reference frame. Finally, the translation system determines a third relationship between the external reference frame and the feature reference frame, whereby the feature sensor is calibrated with respect to the external reference frame.
Using the calibration system of the invention, it is easy to calibrate a feature sensor. The reference indicia are first illuminated in a pattern that is observed by the sensor array and used by the coordinate translation system to calibrate the sensor array to the external reference frame. Next the target is placed within the field of view of the sensor array and the feature sensor under calibration. The portable reference target is calibrated with respect to the reference frame of the sensor array, by using the sensor array to observe the pattern of point light sources situated at the vertices of the portable reference target. The feature sensor is then calibrated by projecting structured light from the feature sensor onto the portable reference target. The structured light intersects the target, producing reflected light patterns at the edges of the target that are then read by the feature sensor. The coordinate translation system then performs the appropriate coordinate translation to map the reading of the feature sensor back to the external reference frame.
The entire calibration sequence can be performed quite quickly. The sensor array and portable reference targets are both lightweight and easily positioned. Moreover, the entire calibration sequence may be performed rapidly under computer control. In most instances, all the calibration technician must do is point the feature sensor in the right direction and at the right orientation, place the portable reference target in front of the feature sensor and then allow the system to do the rest.
To make it easy for the technician to position the feature sensor in approximately the correct direction, the present invention provides a unique "virtual image" positioning aid. The calibration system is preprogrammed with information describing the correct feature sensor mounting angle and position. Mounting the portable reference target to the feature sensor allows the calibration sensor array to track the position of the feature sensor as the calibration technician moves it into the correct position.
The calibration system provides the technician with a computer-generated virtual image of the correct sensor position as well as the actual sensor position. By watching the virtual image on the computer screen, the technician manipulates the feature sensor until the virtual images coincide. Adjusting the physical position of the feature sensor to produce coincident virtual images on the computer screen ensures that the feature sensor is properly pointed in the correct direction and oriented properly. Once pointed in this direction, the coordinate translation system performs the precise sensor calibration, determining the proper coordinate vector manipulation needed to map the feature sensor reference frame to the external reference frame.
The presently preferred implementation uses a virtual image display that is not bilaterally symmetrical (such as the letter "P" or the letter "F") so that the user is presented with an unambiguous symbol with which to position the feature sensor. Advantageously, the positioning system provides real time visual feedback to the calibration technician, making sensor positioning a simple matter.